In continuous ink jet printers an upper print head assembly receives ink under pressure and discharges it to form droplet streams directed generally toward a print zone. In "binary" printers of this type, there usually is provided an array of orifices which produce a plurality of ink streams that are simulated (e.g. by predeterminedly vibrating the orifice plate) to cause the streams to break up into uniformly sized and spaced droplets.
The breakup of each ink stream occurs adjacent a lower print head assembly charge plate, which comprises a plurality of separate charge electrodes, one for each ink drop stream. Charge is provided to drops according to information signals that control periodic voltage energizations of the charge electrodes. Charged drops are usually deflected to a lower print head catcher assembly, e.g. by a deflection electrode field intermediate the charge electrodes and catcher.
In order to assure accurate drop charging, it is important that the charge electrodes be clean and dry. Accumulation of ink or moisture on the charge plate can cause electrical shorting between adjacent electrodes and otherwise impair proper drop charging. Various techniques have been utilized to avoid liquid accumulations on the charge plate surfaces, including special start-up and shutdown procedures designed to control ink flows to not contact those surfaces.
Commonly assigned U.S. Pat. Nos. 4,591,870 and 4,622,562 describe approaches for cleaning the critical charge plate surfaces and assuring that ink and other moisture does not condense on them during printing operations. More particularly, the '870 patent describes a print head start-up routine wherein cycles of ink washing, air skiving and condensate washing are effected to periodically clean critical surfaces on the lower print head assemblies. The teachings in both patents point out that when ink droplets are formed rapidly and in large numbers proximate the lower print head surfaces, ink solvent from the ink streams, or moisture from entrained air, can condense on the charge plate surfaces and cause shorting. To remove such condensate and avoid further condensation, the '562 patent provides a resistance heater component that is adhered, by heat conductive adhesive, in a cavity that is machined into a top, interior portion of the catcher body. The catcher is formed of heat conductive material, e.g. stainless steel or filled plastic. After assembling the heater in the catcher, a molded charge plate assembly is mounted on the top of the catcher body in a heat transfer relation with the resistive heater such that the critical surfaces of the charge plate can be heated. While the construction of the '562 lower print head assembly functions well, it is not simple to fabricate. The heater element must be encapsulated, mounted into the catcher and connected to a heater power source. This involves procurement and handling of a number of discrete parts and their time consuming assembly.